Incident illumination apparatus for microscopes are known which have a common light source for bright-field illumination and dark-field illumination and are configured in accordance with one of the following considerations (A) and (B) below.
(A) Where a good quality of the dark-field illumination and a reduced need for complex optical elements are of primary importance, a more or less inadequate bright-field illumination is accepted. Such an inadequate bright-field illumination is associated with non-uniform or incomplete illumination of the object or the pupil. Reduced contrast due to reflected or parasitic light is also a condition of inadequate bright-field illumination.
An illumination apparatus configured according to these considerations is described, for example, in the book by Michel entitled "Die Mikrophotographie", Springerverlag Vienna, 1957, pages 411 and 412. The known apparatus permits rapid switching from bright-field to dark-field illumination by operating a slider by means of which an annular diaphragm with an ancillary lens is introduced into the beam path. However, the optical system used in this illumination apparatus is not suitable for meeting the Kohler illumination requirements. For this purpose, it is necessary to exchange the entire illumination apparatus which requires much time since the exchange involves also adjustment procedures.
(B) Where a correct bright-field illumination is the primary consideration, compliance with the Kohler illumination principle in the bright-field illumination system, that is, the generation of intermediate images, is required. The light source is imaged for a first time through a collector lens into an aperture iris diaphragm, which can be centered, and through one or more lenses together with the aperture diaphragm into the pupil of the objective serving as condenser. At the same time, an intermediate image has to be formed of the collector with aid of ancillary lenses into the illuminating field iris diaphragm which can likewise be centered. The collector is conjugated with respect to the object plane. Combined bright-field and dark-field illumination apparatus configured according to this principle are described, for example, in German published patent application DE-OS No. 2,941,676, German published and examined patent application DE-AS No. 2,021,784, German patent No. 2,331,750, German published patent applications DE-OS No. 2,542,075, DE-OS No. 2,925,407 and DE-OS No. 3,028,418. In all these illumination apparatus, the light source and a major part of the Kohler optical illumination system are utilized jointly for both bright-field and dark-field illumination. This configuration, however, necessitates a considerable structural length, which is not absolutely necessary for a dark-field illumination since controllable illuminating field diaphragms and aperture diaphragms may be dispensed with and since the light source, which should be as large and as homogeneous as possible, may be imaged into the objective.
When combining the two illumination systems in an optical system common to both, the optical lengths of the bright-field and dark-field illumination apparatus must be identical; also, the optical elements common to both systems require relatively wide apertures, although these are not necessary for a bright-field illumination. Consequently, such combined illumination apparatus have a complicated configuration, and are awkward to operate or, if it is desired to facilitate their operation, additional mechanisms for coupling the switching processes on different optical elements are required. As a result, the optical system of neither one of the two types of illumination can be optimized, but has to be considered as a compromise between the contradictory demands made by the two types of illumination on the optical system required for realization.
German patent Nos. 569,884 and 603,324 disclose arrangements for a rapid change between dark-field and bright-field illumination wherein the bright-field reflector is moved in the course of the switching action by means of which the bright-field illumination beam is reflected into the observation beam path. However, these known arrangements, too, are connected downstream of a single optical system used jointly in both dark-field and bright-field illumination. When switching over, only different coaxial partial bundles are masked out of the common beam path. Accordingly, it is not possible in these known arrangements to optimize the bright-field and the dark-field illumination independently of one another.
German published patent applications DE-OS No. 2,301,597, DE-OS No. 3,200,662 and DE-OS No. 1,622,989 disclose incident-light dark-field illumination apparatus which include ring-shaped fiber bundles placed in surrounding relationship to the objective of the microscope. These ring-shaped fiber bundles receive their light from a second supplementary light source. However, such illumination apparatus are invariably tailored to the needs of a specific objective and cannot be simply exchanged. If the microscope used is equipped with a turret and is to be frequently switched between different objectives, such additional apparatus for dark-field illumination are unsuitable.